The invention is based on a circuit for operating an electric motor, in which the current flowing through the motor is predetermined, the circuit having a monitoring arrangement for ascertaining an overload state of the motor, the monitoring arrangement comparing the motor voltage to a motor voltage limit value that is predetermined by a limit value generator and emitting an overload signal when the motor voltage limit value is not met.
A method of operating an electric motor in which the motor power can be changed as a function of the presetting of a nominal value is known from DE-A 41 29 086. If the nominal value is below a predeterminable threshold, corresponding to low motor power, the voltage applied to the motor is regulated. If the nominal value is above the threshold, corresponding to a higher motor power, the current flowing through the motor is regulated. The known method is supplemented by safety devices. Examples of safety devices include overcurrent shutoff and short-circuit shutoff. In overcurrent shutoff or overcurrent limitation of the current flowing through the electric motor, the actual current value signal emitted by a current sensor is supplied to an overcurrent limiter, which compares the signal to an internally-predeterminable threshold value. After a threshold is exceeded, the overcurrent limiter transmits a switching signal to a selection circuit to suppress the control signal for an end or drive stage. The shutoff signal can also be fed to the motor-drive circuit or the current regulator.
Depending on the type of application, it may occur that a limitation of the current flowing through the electric motor to a fixed, predetermined maximum value does not suffice to provide the used components with permanent protection against thermal overload.